1. Field of the Invention
The invention relates to a three-phase boost-buck power factor correction (PFC) converter. Particularly, the invention relates to a converter using three independent single-phase boost-buck PFC circuits.
2. Description of Related Art
In the past two decades, power electronic technology has been rapidly developed, and various power electronic devices are widely used in chemical industry and communications, etc., in which a rectifier is a most typical one. The conventional rectifiers include diode rectifiers and phase-controlled rectifiers using thyristors. Using as a typical non-linear circuit, in operation, an input current of the rectifier contains a large amount of harmonic component, which may cause a severe harmonic pollution to the public utility grid. A power factor correction (PFC) converter can greatly reduce the harmonic component of the input current to achieve unit power factor rectification, which draws attentions of scholars from various nations.
Presently, a single-phase boost type PFC converter having a boost function is widely used. Such solution has advantages of simple circuit structure, continuous input current and small filter volume, etc., though an application scope thereof is limited, namely, it is only adapted to occasions when an output voltage is greater than an input voltage peak. In some cases, the output voltage is smaller than the input voltage peak, which means that within a fundamental cycle, the converter not only has a phase of working in a boost mode but also has a phase of working in a buck mode. Therefore, the PFC converter that can work in both of the boost mode and the buck mode has become one of the major subjects studied by scholars all over the world.
FIG. 1 is a circuit topology of a conventional single-phase boost-buck PFC converter. The converter can work in the buck mode or the boost mode by controlling the switches S1 or S2. When the switch S1 is constantly turned on and the switch S2 is in a pulse width modulation (PWM) switch working state, the converter is in the boost mode, and when the switch S2 is constantly turned off, and the switch S1 is in the PWM switch working state, the converter is in the buck mode. This circuit is a single-phase boost-buck converter which is only adapted to small power applications.
FIG. 2 is a circuit diagram of an existing two-stage three-phase boost-buck PFC converter. The converter is composed of a front-stage three-phase buck PFC converter and a back-stage boost circuit, which is a three-phase three-wire structure. The three-phase input currents of the circuit are coupled to each other, which is complicated in control, and is of no avail for reducing a total harmonic distortion (THD) of the input current.
FIG. 3 is a circuit diagram of an existing three-level three phase boost-buck PFC converter of a three-phase four-wire structure. Regarding each phase branch, a half branch is in a working state in either a positive or a negative half cycle of a supply voltage, and a working mode thereof (the buck or boost mode) is determined by a relationship between the input voltage and the output voltage. When a polarity of the phase voltage is positive, the upper branch of each phase branch is in the working state. Now, if the phase voltage is greater than the output voltage, the upper branch works in the buck mode. Otherwise, it works in the boost mode. When the polarity of the phase voltage is negative, the lower branch of each phase branch is in the working state. Now, if an absolute value of the phase voltage is greater than the output voltage, the lower branch works in the buck mode. Otherwise, it works in the boost mode. Such circuit effectively resolves the problems of narrow application scope and complicate control of the conventional technique, and avails reducing the total harmonic distortion of the circuit. However, according to the circuit topology, it is known that in the buck mode, two diodes have conduction loss at any time, though in the conventional buck PFC converter, only one diode has the conduction loss during a period when the switch is turned off. Therefore, when the circuit topology of FIG. 3 is used, the more proportion the buck mode occupies, the greater system loss caused by the diode conduction loss is, which may significantly reduce the system efficiency. In the boost mode, one diode has the conduction loss in an inductor energy storage phase, and two diodes have the conduction loss in a freewheeling phase, while in the conventional boost PFC converter, only one diode has the conduction loss in the freewheeling phase, so that efficiency of the converter is reduced.